Lead-free halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. Recently, lanthanide ion (Ln³⁺) with rich electronic energy levels have been proposed for tailoring the optical performances of DPs toward the near-infrared (NIR) regions. 

Among these DPs, Cs₂Na(Ag)InCl₆ DPs have been widely reported as one of the excellent hosts for Ln³⁺ doping owing to the direct bandgap character and high chemical stability. However, their local electronic structures remain essentially untouched, which restricts the development of Ln³⁺-doped DPs. 

In a study published in Advanced Science, the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences introduced Yb³⁺ into Cs₂NaInCl₆ DPs and realized efficient NIR luminescence with the optimal photoluminescence quantum yields (PLQY) of 39.4%. 

The researchers unveiled the local electronic structure of Cs₂NaInCl₆:Yb³⁺ through density functional theory calculation and Bader charge analysis, which indicated that electrons in [YbCl₆]^3- octahedron were strongly localized in Cs₂NaInCl₆:Yb³⁺, while they were delocalized towards Ag⁺ in Cs₂AgInCl₆:Yb³⁺. Such localized electron can effectively boost the NIR luminescence via Cl^--Yb³⁺ charge transfer sensitization in Cs₂NaInCl₆.  

Benefiting from the localized electrons of [YbCl₆]^3- octahedron in Cs₂NaInCl₆ DPs, an efficient strategy of Cl^--Yb³⁺ charge transfer sensitization is proposed to obtain intense NIR luminescence of Ln³⁺.  

The researchers demonstrated the proposed novel sensitization strategy for enhancing the NIR emission of Ln³⁺ to be superior to the self-trapped excitons sensitization in the well-established Cs₂AgInCl₆ counterparts. 

They carried out temperature-dependent steady-state and transient PL spectroscopic measurements to verify the Cl^--Yb³⁺ charge transfer process in Cs₂NaInCl₆:Yb³⁺ by the characteristic transition from charge transfer band (CTB) to ²F_7/2 (Yb³⁺) and ²F_5/2 (Yb³⁺). 

Density functional theory calculation and Bader charge analysis indicated that the [YbCl₆]^3- octahedron is strongly localized in Cs₂NaInCl₆:Yb³⁺, which facilitates the Cl^--Yb³⁺ charge transfer process.  

Furthermore, the researchers achieved efficient NIR luminescence from Er³⁺ with PLQY of 7.9% in Yb³⁺/Er³⁺ co-doped Cs₂NaInCl₆ DPs due to the energy transfer from Cl^--Yb³⁺ CTB to Er³⁺. 

These findings provide a general approach to achieving effective NIR emission of Ln³⁺ in halide DPs, opening up a new avenue for exploring NIR-emitting perovskite derivatives toward versatile applications. 

Schematic of electronic structure and luminescence mechanism in Cs₂NaInCl₆:Ln³⁺ double perovskites (Image by Prof. CHEN's group) 

Contact:   

Prof. CHEN Xueyuan  

Fujian Institute of Research on the Structure of Matter   

Chinese Academy of Sciences   

Email: xchen@fjirsm.ac.cn